Cabin windows that are conventionally used in modern pressurized passenger aircraft typically have a double-pane construction including two window panes spaced apart from one another with a hollow interspace therebetween, whereby the double panes sometimes are intended to provide fail-safe redundancy.
FIG. 1 schematically represents a conventional double-pane cabin window 1' comprising an inner window pane 2' and an outer window pane 3' which are spaced apart from one another and held around their outer perimeter edges in a window seal 8' mounted in a window frame 11' provided on the outer fuselage skin 12' of the aircraft. A hollow interspace is formed between the two panes 2' and 3'. In order to achieve a pressure equalization in this window interspace during flight, and especially during the climb and descent phases of the flight, a ventilation hole 5' is provided at or near the bottom of the inner window pane 2'. This ventilation hole 5' provides an opening from the pressurized passenger cabin into the window interspace. Thus, as a result of any pressure difference between the pressurized cabin interior and the window interspace, an airflow or air exchange between the cabin interior and the window interspace will take place.
Namely, if the pressure in the cabin is greater than the pressure the window interspace, then air will flow from the cabin into the window interspace and achieve a pressure equalization.
Since the outer window pane 3' generally is subjected to a temperature below -30.degree. C. during flight, even the smallest amount of water vapor present in the cabin air that flows into the window interspace will condense of the inner surface of the outer window pane 3'. Of course, greater condensation will occur as the relative humidity of the interior cabin air increases, which is dependent upon the flight condition of the aircraft, and whereby operation of passenger service galleys, the number of passengers, the duration of the flight, and the location and climatic conditions of the flight, are all factors that play a part in the proportion of water vapor in the cabin air. The water vapor laden cabin air will pass through the ventilation hole 5' and thus pass into the window interspace. Especially in situations when the relative humidity of the air is higher than usual, for example in the event of high occupancy of the passenger aircraft, then it is particularly likely that the inner surface of the outer window pane 3' will become fogged by condensation, which may even form a layer of frost or ice due to the very cold external temperatures during flight. As the aircraft then descends into warmer air strata, the frost or ice layer within the window interspace will melt, forming fog or water droplets on the window panes, and in the extreme situation even causing the accumulation or puddling of condensed water at the bottom of the window interspace.